refactor(07-03): enhance JSDoc comments for memoized selectors

- Add comprehensive JSDoc with usage descriptions for all 8 selectors
- Document memoization behavior (when selectors recompute)
- Add @private tags to input selectors
- Document return value types and defaults (empty arrays, 0, idle states)
- Clarify channel ID parameter (session ID, lesson ID, or 'global')
- All 68 tests still passing after refactoring

Part of Phase 7 Plan 3 (WebSocket Integration & Selectors)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/store/features/sessionChatSlice.js

@@ -311,17 +311,32 @@ export const {
 // Phase 7 Plan 3: Memoized selectors using createSelector from Redux Toolkit
 import { createSelector } from '@reduxjs/toolkit';
 
-// Input selectors (direct state access)
+/**
+ * Input selectors for memoized selectors
+ * These directly access state and are used as inputs to createSelector
+ */
+
+/** @private */
 const selectSessionChatState = (state) => state.sessionChat;
+
+/** @private */
 const selectMessagesByChannel = (state) => state.sessionChat.messagesByChannel;
+
+/** @private */
 const selectUnreadCounts = (state) => state.sessionChat.unreadCounts;
+
+/** @private */
 const selectFetchStatusMap = (state) => state.sessionChat.fetchStatus;
 
 /**
  * Select messages for a specific channel
+ *
+ * Returns array of messages for the given channel, or empty array if channel doesn't exist.
+ * Memoized with createSelector - only recomputes when messagesByChannel or channel changes.
+ *
  * @param {Object} state - Redux state
- * @param {string} channel - Channel ID
- * @returns {Array} Messages for the channel
+ * @param {string} channel - Channel ID (session ID, lesson ID, or 'global')
+ * @returns {Array} Messages for the channel, sorted chronologically
  */
 export const selectChatMessages = createSelector(
   [selectMessagesByChannel, (state, channel) => channel],
@@ -330,9 +345,13 @@ export const selectChatMessages = createSelector(
 
 /**
  * Select unread count for a specific channel
+ *
+ * Returns unread count for the given channel, or 0 if channel doesn't exist.
+ * Memoized with createSelector - only recomputes when unreadCounts or channel changes.
+ *
  * @param {Object} state - Redux state
- * @param {string} channel - Channel ID
- * @returns {number} Unread count for the channel
+ * @param {string} channel - Channel ID (session ID, lesson ID, or 'global')
+ * @returns {number} Unread count for the channel (0 if none)
  */
 export const selectUnreadCount = createSelector(
   [selectUnreadCounts, (state, channel) => channel],
@@ -341,8 +360,12 @@ export const selectUnreadCount = createSelector(
 
 /**
  * Select total unread count across all channels
+ *
+ * Sums unread counts from all channels for badge display.
+ * Memoized with createSelector - only recomputes when unreadCounts changes.
+ *
  * @param {Object} state - Redux state
- * @returns {number} Total unread count
+ * @returns {number} Total unread count across all channels
  */
 export const selectTotalUnreadCount = createSelector(
   [selectUnreadCounts],
@@ -350,9 +373,13 @@ export const selectTotalUnreadCount = createSelector(
 );
 
 /**
- * Select whether chat window is open
+ * Select whether chat window is currently open
+ *
+ * Used to control chat window visibility and unread increment logic.
+ * Memoized with createSelector - only recomputes when sessionChat state changes.
+ *
  * @param {Object} state - Redux state
- * @returns {boolean} Is window open
+ * @returns {boolean} True if chat window is open
  */
 export const selectIsChatWindowOpen = createSelector(
   [selectSessionChatState],
@@ -360,9 +387,13 @@ export const selectIsChatWindowOpen = createSelector(
 );
 
 /**
- * Select active channel ID
+ * Select currently active channel ID
+ *
+ * Identifies which channel is currently being viewed in the chat window.
+ * Memoized with createSelector - only recomputes when sessionChat state changes.
+ *
  * @param {Object} state - Redux state
- * @returns {string|null} Active channel ID
+ * @returns {string|null} Active channel ID, or null if none selected
  */
 export const selectActiveChannel = createSelector(
   [selectSessionChatState],
@@ -371,9 +402,14 @@ export const selectActiveChannel = createSelector(
 
 /**
  * Select fetch status for a specific channel
+ *
+ * Returns loading state for message history fetch operations.
+ * Defaults to 'idle' if channel has never been fetched.
+ * Memoized with createSelector - only recomputes when fetchStatus or channel changes.
+ *
  * @param {Object} state - Redux state
  * @param {string} channel - Channel ID
- * @returns {string} Fetch status ('idle', 'loading', 'succeeded', 'failed')
+ * @returns {string} Fetch status: 'idle' | 'loading' | 'succeeded' | 'failed'
  */
 export const selectFetchStatus = createSelector(
   [selectFetchStatusMap, (state, channel) => channel],
@@ -381,9 +417,13 @@ export const selectFetchStatus = createSelector(
 );
 
 /**
- * Select send status
+ * Select send message status
+ *
+ * Returns loading state for the current message send operation.
+ * Memoized with createSelector - only recomputes when sessionChat state changes.
+ *
  * @param {Object} state - Redux state
- * @returns {string} Send status ('idle', 'loading', 'succeeded', 'failed')
+ * @returns {string} Send status: 'idle' | 'loading' | 'succeeded' | 'failed'
  */
 export const selectSendStatus = createSelector(
   [selectSessionChatState],
@@ -392,8 +432,12 @@ export const selectSendStatus = createSelector(
 
 /**
  * Select send error message
+ *
+ * Returns error message if the last send operation failed.
+ * Memoized with createSelector - only recomputes when sessionChat state changes.
+ *
  * @param {Object} state - Redux state
- * @returns {string|null} Send error message
+ * @returns {string|null} Error message, or null if no error
  */
 export const selectSendError = createSelector(
   [selectSessionChatState],